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48 Project Definition 7.1 Roadway and Geometrical The initial phase of use for the Recommended Practice is to define the project details. The fundamental roadway and geo- metrical parameters are compiled so that they are available for use in the design evaluations completed in Phases 2 and 3. The following are the recommended roadway and geo- metrical design parameters: â¢ Unique project, bid, or agency-wide identifier â¢ Type of installation or pipe function (culvert or storm sewer) â¢ Location â¢ Roadway functional classification â¢ DSL â¢ Culvert length â¢ Minimum fill height â¢ Maximum fill height â¢ Maximum size (considering vertical and lateral conflicts) â¢ Minimum size (considering maintenance, future rehabili- tation) â¢ Upstream invert elevation â¢ Downstream invert elevation â¢ Design slope â¢ Skew â¢ Breaks in slope or alignment â¢ Installation condition (embankment/trench) 7.1.1 DSL The general principle for the use of a DSL-based system is that the higher the road classification and the higher the consequences from premature failure of a drainage system, then the longer the DSL should be. Other factors that could influence the DSL would be the height of embankment fill above the pipe which can necessitate full road closures to allow pipe replacements. Typically agencies use DSL values of 25, 50, 75 and 100 years, with design lives of 75 and 100 years being reserved for high volume freeways, and 25 years being used for entrance culverts and similar pipes. 7.2 Hydrology and Waterway The fundamental hydrologic, waterway, and hydraulic parameters are compiled in this stage for use in the design evaluations completed in Phases 2 and 3 of the Recommended Practice. At least one baseline hydraulic design for the drainage application being evaluated is also required to be undertaken outside of the Recommended Practice to provide a starting point for the Phase 2 and 3 design evaluations of available alternatives. The basic hydrologic design parameters are as follows: â¢ Drainage area â¢ Design flow rate â¢ Design storm â¢ Check storm â¢ Allowance for future watershed changes A hydrological analysis to define the drainage system flow requirements is required to be completed outside of the Recommended Practice, typically using procedures outlined in the most recent version of the following documents: â¢ FHWA Highway HydrologyâHDS â¢ AASHTO Model Drainage Manual In addition, hydraulic design parameters such as those listed below need to be defined, with guidance provided in FHWA Hydraulic Design of Highway CulvertsâHDS 5: â¢ Allowable headwater criteria â¢ Minimum allowable flow velocity C H A P T E R 7
49 â¢ Maximum allowable flow velocity â¢ Joint rating: soil tight, silt/fines tight, or water tight â¢ End treatments â¢ Section variations (e.g., bends, junctions, wyes, transitions) â¢ Aquatic organism passage requirements Two default options for baseline pipe roughness categories are defined below, noting that agencies may use alternate default category names and representative minimum Manningâs n values as preferred. The recommended default pipe roughness categories are as follows: â¢ Smooth (n = 0.012) â¢ Corrugated (n = 0.024) If a more rigorous category classification scheme is desired, the following four categories are recommended: â¢ Ultra smooth (n = 0.009) â¢ Smooth (n = 0.012) â¢ Corrugated (n = 0.024) â¢ Structural plate (0.036) The baseline hydraulic design for each of the generic base- line pipe roughness categories is performed through use of the FHWA HY-8 Hydraulic Analysis Program or other means. In the absence of minimum flow requirements and other special hydraulic considerations, the classification of the drainage application as inlet or outlet controlled can be used to streamline the baseline hydraulic evaluations. Starting with analysis of the roughest category first, if the system is found to be inlet controlled, all smoother baseline categories can be set to that same size without requiring independent analysis. If an evaluation results in outlet controlled conditions, the next roughness category evaluation would be completed to determine the potentially smaller baseline pipe size require- ments for that roughness condition. It is noted that variations in barrel roughness impact outlet velocity and thus outlet scour, potentially resulting in variations to the required engineering controls at the outlet to achieve equivalent performance. 7.3 Environmental and Geotechnical Collection of site-specific environmental and geotechnical data from the native soil, backfill, flow, and groundwater is necessary to estimate the material service life of drainage pipe systems. 7.3.1 Definition of Site Environmental Parameters Data on the soil, backfill, and water should be collected in accordance with the most recent versions of the following standards: â¢ Soil pH: AASHTO T 289 or ASTM G51 â¢ Water pH: ASTM D1293 or ASTM D5464 â¢ Soil resistivity: AASHTO T 288 â¢ Water resistivity: ASTM D1125 â¢ Chloride concentration: AASHTO T 291 or ASTM D512 â¢ Sulfate concentration: AASHTO T 290 or ASTM D516 â¢ Flow rate: ASTM D3858 or ASTM D5243 Relevant standardized test procedures adopted by state transportation agencies may also be used to collect site-specific environmental data. Alternatively, many agencies make use of field kits that are specifically designed for this purpose and are useful in supplementing the data from laboratory testing. Data collected at a single location at a specific time may not be representative of conditions that exist at a site over the lifetime of the drainage pipe system. To account for poten- tial seasonal and other variations in water characteristics, collection of environmental data at multiple locations and at multiple times during the year should be considered depend- ing on the scale of the project. Changes in surrounding land use (e.g., fertilizer impacted runoff from nearby agricultural lands, roadway salting efforts in the winter) and flow charac- teristics should also be considered. Test values can be seasonally affected by such factors as rain- fall, flooding, drought decaying vegetation, and man-made influences (e.g., fertilizer or road salt runoff). Whenever pos- sible, environmental tests should be taken during periods con- sidered representative of critical environmental conditions. In addition to the collection of soil and water environ- mental data to allow completion of the quantitative durability evaluations in Phase 2, it is strongly recommended that the in-service performance of nearby drainage systems be recorded and used to back-calculate estimated environmental conditions for the observed service life conditions through reverse appli- cation of the methods discussed in Phase 2. Based on comparison of the field measured and back- calculated environmental conditions the designer would choose the critical value in each category to bring forward through the remainder of the Recommended Practice. 7.3.2 Geotechnical Information A geotechnical investigation should be performed in accordance with AASHTO Standard Recommended Practice R13-03 âConducting Geotechnical Subsurface Investigationsâ
50 and agency-specific guidance. It is convenient to include the geotechnical data needed for drainage system design in the scope of a pavement rehabilitation investigation. The focus of the investigation for drainage system design should be on the following: â¢ Determining the ground conditions that will act in sup- port of the drainage pipe system â¢ Determining the suitability of native materials to be used in construction â¢ Compaction characteristics of construction materials â¢ Potential for abrasive bedload to be generated from water- shed soils â¢ Collecting samples for the testing listed in Section 7.5.2 of the Recommended Practice 7.3.3 Additional Considerations Additional design drivers are considered in Phase 1D and may cause the drainage application to be designed outside of the Recommended Practice, and/or for additional design constraints to be placed on the technical evaluations. â¢ Earthquake hazards including liquefaction, fault crossings, and so forth â¢ Ecological factors upstream, downstream, or within the culvert â¢ Minimum and maximum temperatures, and resulting extreme temperature impacts â¢ Ground freezing and other cold weather considerations â¢ High maximum temperatures can impact the material ser- vice life of thermoplastics and other pipe materials and may require special design considerations â¢ Erosion and scour potential â¢ Fire risk and consequence â¢ Roadway chemical spill risk and consequence â¢ Other geologic, environmental, or man-made conditions Assign an abrasion level for use during Phase 2 by using the data collected in Phases 1B and 1C. 7.4 Inventory of Available Pipe Systems Following completion of Phases 1A through 1D, the design engineer should refer to the agency inventory of available and approved pipe systems to set the listing of pipe systems to be included in the matrix and to be evaluated as part of the cur- rent application of the Recommended Practice. This Recom- mended Practice phase is included to promote recording of the inventory used in the Recommended Practice for a given project or drainage application as agency inventories will likely change over time. The range of sizes evaluated in each application of the Recommended Practice should be sufficient to capture all suit- able alternative pipe systems, but be limited to those systems that are practical for a given drainage application. It may be beneficial to evaluate pipe systems within two sizes above the baseline designs during application of the Recommended Practice so as to increase the bidding options. The evaluation of non-circular shapes (pipe arch, hori- zontal elliptical, vertical elliptical) is not required for many standard drainage applications. However, these alternate shapes and/or the use of multiple barrels are common prac- tice and require evaluation when applicable. It is recom- mended that the potential need for non-circular shapes be identified during Phase 1 through evaluation of the baseline hydraulic design and the roadway and geometrical data, with alternate shapes included in the Recommended Practice evaluations if it is determined that non-circular shapes may be required. In line with standard design practice, if the Phases 1A through 1D evaluations do not identify a potential need to use non-circular shapes, it is recommended that these shapes not be evaluated to simplify and streamline the implementation of the Recommended Practice. Multiple barrel drainage systems can be evaluated using the Recommended Practice through analysis of an individual component with the Recommended Practice process, noting that the chosen option (size and number of barrels) must meet the geometric constraints defined in Phase 1A.